Backlights for liquid crystal displays (LCDs) based on Cold Cathode Fluorescent Lamps (CCFL) are known that have a high lumen efficacy of 50–60 lm/W. This high lumen efficacy is due to the unique properties of mercury vapor. However, due to environmental constraints the display industry is forced to develop alternative backlights without using mercury.
CCFL backlights which are used in large area TVs have an unacceptably thick direct illumination package, which increases the thickness of flat panel televisions. A CCFL backlight for scanning or scrolling with an adequate duty cycle that effectively suppresses motion blur is very difficult to achieve for commercially acceptable televisions.
An alternative backlight uses a flat xenon discharge lamp, having a lumen efficacy of ˜18 lm/W. Thus, efficiency is sacrificed for the elimination of the hazards of mercury.
Field Emission Displays (FED) are expensive, and commercial production of FEDs based on Spindt type field emitters is very limited. Televisions differ from computer displays, because televisions primarily display moving images and not merely text, still pictures and graphic images. LCDs suffer from motion blurring due to the sample-hold effect. Field emission backlight devices are well suited for scrolling or scanning of pixels, which reduces the motion blurring caused by the sample-hold effect.
The parameters presented in Table 1 are goals based on the performance of conventional cathode ray tubes (CRTs), which support high average luminance and high dynamic range. For example, the factor between peak luminance of CRTs may be as great as factor of six. In conventional liquid crystal televisions the average luminance is lower, because LCDs usually have better contrast than CRTs. Also, the dynamic range is usually lower. So, a peak luminance of 8,000–10,000 cd/m2 is a desirable objective for a liquid crystal television backlight. In this case 16 lamps of 375 lumens each are required for a 32″ wide-screen, liquid crystal television.
Furthermore, scanning or blinking backlights are used to reduce motion blur in LCD's, the lumen requirement is much larger. In Table 1 it is indicated that that for a 10% duty cycle 400 lamps would be required, which is commercially impractical.